The semiconductor industry continues to strive for improvements in the speed and performance of semiconductor devices. Strained silicon technology has been shown to enhance carrier mobility in both n and p-channel devices, and thus has been of interest to the semiconductor industry as a means to improve device speed and performance. Currently, strained silicon layers are used to increase electron mobility in n-channel CMOS transistors. There has been research and development activity to increase the hole mobility of p-channel CMOS transistors using strained silicon germanium layers on silicon.
It was realized that thin strained layers could be made with larger lattice mismatches and much more strain before yield than was possible with thick bulk samples. This lead to the development of structures with thin silicon germanium layers on silicon to take advantage of the higher hole mobility in germanium. These structures require a silicon capping layer over the silicon germanium layer to prevent incorporation of germanium into the gate oxide insulator, which lead to the development of transistors with enhanced hole mobility using dual channel structures with strained silicon and strained silicon germanium.
FIG. 1A illustrates a known device for improved hole mobility with an n-type silicon substrate 101, a silicon germanium layer 102, a silicon capping layer 103, a gate oxide 104, a gate 105, and N+ source/drain regions 106 and 107. FIG. 1B illustrates a band structure for the device of FIG. 1A, and indicates that some carriers or holes are at the silicon-oxide interface and some are confined in the silicon germanium layer. Both the silicon germanium and the silicon capping layers will be strained if they are thin. Alternatively, the silicon germanium layer may be graded to a relaxed or unstrained layer resulting in more stress in the silicon cap layer. One process for forming silicon over silicon germanium (Si/SiGe) structures uses a relatively expensive ultra high vacuum chemical vapor deposition (UHVCVD) process. The germanium content is graded in steps to form a fully relaxed silicon germanium buffer layer before a thin (e.g. ˜20 nm) strained silicon channel layer is grown. A lower cost implantation process has been disclosed in US 2004/0221792 entitled Strained Si/SiGe Structures By Ion Implantation.” As disclosed in US 2004/0221792, germanium ions can be implanted into a silicon substrate with a desired dose and energy to at least partially amorphize the surface silicon layer. The substrate is heat treated to regrow a crystalline silicon layer over a resulting silicon germanium layer using a solid phase epitaxial (SPE) process. The crystalline silicon layer is strained by a lattice mismatch between the silicon germanium layer and the crystalline silicon layer.
More recently, strained silicon layers have been fabricated on thicker relaxed silicon germanium layers to improve the mobility of electrons in NMOS transistors. For example, strained silicon layers along the side of relaxed silicon pillars for use in logic circuits have been disclosed (see U.S. Pat. No. 6,900,521 entitled “Vertical Transistors and Output Prediction Logic Circuits Containing Same”) and ballistic injection of electrons from high mobility strained silicon layers onto the floating gates of flash memories or into the charge storage regions of NROM Memory devices have been disclosed (see US 2005/0212065 entitled “NROM Memory Device With A High-Permittivity Gate Dielectric Formed By the Low Temperature Oxidation Of Metals,” and US 2005/0247972 entitled “Ballistic Direct Injection NROM Cell On Strained Silicon Structures”).
Structures with strained silicon on silicon germanium on insulators have been described (see, for example, US 2005/0029619 entitled “Strained Si/SiGe/SOI Islands and Process of Making Same”) as well as structures with just strained silicon over a localized oxide insulator region (see US 2004/0217391 entitled “Localized Strained Semiconductor on Insulators”). These structures yield high mobility and high performance transistors on a low capacitance insulating substrate.
Wafer bending has been used to investigate the effect of strain on mobility and distinguish between the effects of biaxial stress and uniaxial stress. Bonding a semiconductor onto bowed or bent substrates has been disclosed to introduce strain in the semiconductor (see US 2005/0020094, entitled “Strained Semiconductor By Full Wafer Bonding”). Stress can also be introduced by wafer bonding (see, for example, US 2004/0224480 entitled “Micromechanical Strained Semiconductor by Wafer Bonding,” US 2004/0217352 entitled “Strained Semiconductor By Wafer Bonding With Misorientation,” and US 2004/0173798 entitled “Micro-Mechanically Strained Semiconductor Film.” Packaging is another technique to introduce mechanical stress by bending.
FIGS. 2-4 illustrate known techniques to strain channels and improve carrier mobilities in CMOS devices. FIG. 2 illustrates a known device design to improve electron mobility in NMOS transistors using a tensile strained silicon layer on silicon germanium. As illustrated, a graded silicon germanium layer 208 is formed on a p-type silicon substrate 209 to provide a relaxed silicon germanium region 210, upon which a strained silicon layer 211 is grown. The transistor channel is formed in the strained silicon layer 211. There is a large mismatch in the cell structure between the silicon and silicon germanium layers, which biaxially strains the silicon layer. The biaxial strain modifies the band structure and enhances carrier transport in the silicon layer. In an electron inversion layer, the subband splitting is larger in strained silicon because of the strain-induced band splitting in addition to that provided by quantum confinement. As illustrated in FIG. 3, uniaxial compressive stress can be introduced in a channel 312 of a PMOS transistor to improve hole mobility using silicon germanium source/drain regions 313 in trenches adjacent to the PMOS transistor. Silicon-carbon source/drain regions in trenches adjacent to an NMOS transistor can introduce tensile stress and improve electron mobility. FIG. 4 illustrates a known device design to improve mobility for both NMOS and PMOS transistors using silicon nitride capping layers 414. These silicon nitride capping layers can be formed to introduce tensile stress for NMOS transistors and can be formed to introduce compressive stress for PMOS transistors.